Magnetic storage systems, such as hard disk drives, are used to store large amounts of information. A magnetic head in a magnetic storage system typically includes a read/write transducer for retrieving and storing magnetically encoded information on a magnetic recording medium, such as a disk.
The ability to increase the storage capacity in magnetic recording is an ongoing concern. As the amount of information to be stored on the magnetic recording medium continues to increase, demands for higher-density recording also continue to increase. High areal densities can be obtained in a perpendicular magnetic recording (PMR) system by increasing the linear density (i.e., the number of bits written in the down-track direction) and/or the track density (i.e., the widths of the tracks). Thus, written bits must become narrower (i.e., in the cross-track direction) and shorter (i.e., in the down-track direction).
Writing shorter bits requires a larger write field gradient. Ideally, the write field gradient is a step, that is, an infinite slope of the field gradient, at the dynamic coercivity of the recording medium being used to enable shorter bits to be written. One way to increase the write field gradient is to reduce the write gap, but doing so reduces the write field. Likewise, for conventional perpendicular magnetic recording, the need to increase track density requires the writer to be narrower, which reduces both the write field and the write field gradient.
Thus, recording bits that are both narrower and shorter raises a fundamental problem: narrower bits require a narrower writer, which reduces the write field magnitude and gradient, whereas the shorter bits require larger gradients and at least no degradation in field magnitude. This problem has led to various proposed alternative magnetic recording approaches and technologies, such as shingled magnetic recording (SMR) and heat-assisted magnetic recording (HAMR).
U.S. Pat. No. 7,538,977 proposes to place a diamagnetic material in the write gap. The primary disadvantage of this approach is the absence of readily-available materials that have sufficient performance at room temperatures. A super-conductor may be needed.
Another proposed approach is microwave-assisted magnetic recording (MAMR). In MAMR systems, a spin-torque oscillator (STO) comprising a field-generation layer (FGL) and spin-polarization layer (SPL) is placed within in the write gap. The write head generates a write field that, beneath the main pole, is substantially perpendicular to the magnetic recording layer, and the STO generates a high-frequency auxiliary field to the recording layer. Ideally, the auxiliary field has a frequency close to the resonance frequency of the magnetic grains in the recording layer to facilitate the switching of the magnetization of the grains. As a consequence, the oscillating field of the STO's FGL resonates with the media and provides strong writing despite having a narrow writer.
Although the theory of MAMR is understood, in practice it may be difficult to fabricate a STO structure that has sufficiently stable oscillations at a frequency appropriate for a media, which generally has a narrow range of frequencies to which it responds. For example, in many cases, the FGL's frequency is too low to resonate with the media, or the FGL's frequency is within the proper range to resonate with the media, but the oscillations are unstable. Moreover, the SPL-plus-FGL structure of the STO may be difficult to build into high-gradient, but narrow, write gaps. In addition, the use of MAMR requires a joint optimization of both the writer and the media, which may be complicated, time-consuming, or expensive.
Thus, there is an ongoing need for a narrow, stable, more-easily-fabricated writer that provides adequate write field and gradient to enable high-density magnetic recording without requiring a joint optimization of the writer and media.